1. Field of the Invention
The present invention relates to the field of infrared (IR) photodetection. More particularly, the invention relates to semiconductor IR detectors tailored for multicolor operation.
2. Description of the Prior Art
Solid-state, IR photodetectors often include a semiconductor body that is grown to form a superlattice of some predetermined configuration. Such detectors are fabricated from a variety of techniques such as molecular beam epitaxy (MBE), metal organic vapor phase epitaxy (MOVPE) and derivatives of these two techniques, including combinations of both. Those concerned with the development of such photodetector devices have long recognized the need for improved procedures of tailoring their frequency-absorption characteristics to provide for multicolor operation.
One class of IR photodetectors has a molecular structure characterized by a series of relatively wide quantum wells separated by relatively wide potential barriers. The barriers are usually of sufficient width to prevent any significant coupling between the energy levels of adjacent wells. The location of the ground and excited subbands in these wells are primarily dependent on the width of the wells. In general, such subbands rise when the well width is made narrower and fall when the well width is made wider. Consequently, fabricators of multicolor IR photodetectors often tailor their frequency-absorption characteristics by controlling the size of the well widths. Although such devices have served the purpose, the degree of control that fabricators have in tailoring their absorption characteristics using this technique has proved to be quite limiting.
Kwong-Kit Choi, one of the coinventors of the present invention, describes a more flexible tailoring technique in his U.S. Pat. No. 5,013,918 (hereinafter referred to as the '918 patent), issued May 7, 1991. The '918 patent discloses a multicolor IR photodetector for use in a variety of applications such as fiber optics communication, IR remote control systems, and IR sensing technology. The semiconductor device of the '918 patent has an array of quantum-well pairs. Each quantum-well pair includes a thick well and a thin well separated by a thin barrier that permits strong coupling between the thick and thin wells. Within these wells, there is a ground electron state and a number of excited states with different energies. IR energy incident on the device gives rise to intersubband absorption which excites electrons from the ground state into one of the excited states. A measurable photosignal results when the photoexcited electrons tunnel out of these wells. The IR absorption characteristics of these detectors show multiple absorption peaks. The results achieved using this technique differ substantially from those achieved by simply controlling the well width.
The effects of placing a thin barrier layer in a relatively wide quantum well, as is the situation in the '918 patent, have been the subject of several other studies. The results of two of these studies are reported in the following publications: Peters et al., "New Method of Controlling the Gaps Between the Minibands of a Superlattice", Applied Physics Letters 55(11), Sep. 11, 1989, pp. 1006-1008; Trzeciakowski et al., "Tailoring the Intersubband Absorption in Quantum Wells", Applied Physics Letters 55(9), Aug. 28, 1989, pp. 891-893.
The Trzeciakowski et al. publication, which discusses in detail the use of intersubband transitions for the detection of IR, shows how the transition energy in an IR detector may be tailored by shifting the ground state in the detector quantum well while not appreciably moving its excited state. In this regard, ground-state shifting was achieved by growing a thin layer of Al.sub.x Ga.sub.1-x As in the middle of a GaAs well thereby creating a potential barrier for the electrons and for the holes. According to this publication, the thin barrier pushes the ground subband up while not appreciably effecting the excited subband because the wave function of the latter vanishes in the middle of the well while it is a maximum for the former.
Peters et al. also discuss the thin-barrier technique and shows how the barrier width and height, and the position of the barrier in the quantum well effect the positions of the ground and excited subbands. Peters et al. studied structures in which the well was also made of GaAs and the barrier was made out of the alloy Al.sub.x Ga.sub.1-x As. This study also describes how the IR absorption frequency of a quantum well behaves when the thin barrier of a particular width and height is placed in the center of the well.
Although there has been a long recognized need for IR photodetectors that can be tailored to detect multiwavelength radiation over a very wide range, no practical device for doing so has yet been devised. The present invention fulfills this need.